1. Field of the Invention
The present invention relates in general to optical devices, and it particularly relates to a high numerical aperture (NA) catadioptric focusing device having a flat kinoform phase profile, for use in data storage systems such as optical and magneto-optical (MO) disk drives.
2. Description of Related Art
In a MO storage system, a thin film read/write head includes an optical assembly for directing and focusing an optical beam, such as a laser beam, and an electromagnetic coil that generates a magnetic field for defining the magnetic domains in a spinning data storage medium or disk. The head is secured to a rotary actuator magnet and a voice coil assembly by a suspension and an actuator arm positioned over a surface of the disk. In operation, a lift force is generated by the aerodynamic interaction between the head and the disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height is maintained over a full radial stroke of the rotary actuator assembly above the surface of the disk.
A significant concern with the design of the MO head is to increase the recording or a real density of the disk. One attempt to achieve objective has been to reduce the spot size of the light beam on the disk. The diameter of the spot size is generally inversely proportional to the numerical aperture (NA) of an objective lens forming part of the optical assembly, and proportional to the wavelength of the optical beam. As a result, the objective lens is selected to have a large NA. However, the NA in objective lenses can be 1 if the focusing spot were in air, thus limiting the spot size.
Another attempt to reduce the spot size and to increase the recording a real density has been to use solid immersion lenses (SILs) with near field recording, as exemplified by the following references:
U.S. Pat. No. 5,125,750, titled "Optical Recording System Employing a Solid Immersion Lens". PA1 U.S. Pat. No. 5,497,359, titled "Optical Disk Data Storage System With Radiation-Transparent Air-Bearing Slider". PA1 U.S. Pat. No. 5,689,480, titled "Magneto-Optic Recording System Employing Near Field Optics". PA1 Lee, C.W., et al., "Feasibility Study on Near Field Optical Memory Using A Catadioptric Optical System", Optical Data Storage, Technical Digest Series, Volume 8, pages 137-139, May 10-13, 1998; and PA1 "Parallel Processing", 42 Optics and Photonics News, pages 42-45, June 1998. While this catadioptric SIL system can present certain advantages over conventional SILs, it does not appear to capture the entire incident, collimated beam. This represents a waste of valuable energy that could otherwise increase the evanescent optical field.
Yet another attempt at improving the recording head performance proposes the use of near-field optics, as illustrated by the following reference:
A catadioptric SIL system is described in the following references, and employs the SIL concept as part of the near-field optics:
Other concerns related to the manufacture of MO heads are the extreme difficulty and high costs associated with the mass production of these heads, particularly where optical and electromagnetic components are assembled to a slider body, and aligned for optimal performance.